Remove the hard-coded number of parallel tasks for resolution

[houtanb]

The number of parallel tasks for resolution is hard coded to $50$, with the extra tasks being wound down.

The relevant code line is in src/nightly_update_workflow/worker/main.py#17:

resolve_forecasts = [
    [
        ("func-resolve-forecasts", True, cloud_run.timeout_1h * 3, 50),
    ]
]

This was a short-term hack.

Fix this by:

• counting the number of folders in the forecast sets bucket
• launching that many tasks instead of $50$